1. Field of the Invention
The invention generally relates to high performance bipolar transistors and, more particularly, to a method for making such transistors having self-aligned emitter and base regions. The emitter and collector contacts are made directly to the doped single crystal silicon regions while contact to the base is made via a doped polycrystalline silicon layer formed above the surface of the doped single crystal region.
2. Description of the Prior Art
U.S. Pat. No. 4,160,991, issued to the present inventors on July 10, 1979 for "High Performance Bipolar Device and Method For Making Same" and assigned to the present assignee discloses a method using a polycrystalline base contact covered by an oxidized layer which spaces the emitter contact from the base contact. Thus, the base and emitter spacing is as narrow as the thickness of the oxide coating, which can be reduced to less than about 0.5 .mu.m. Narrow base-to-emitter contact spacing reduces the base resistance of the transistor and significantly enhances switching speed performance.
Although the aforesaid contact spacing is reduced, the emitter region is not self aligned relative to the base region of the transistor fabricated in accordance with the cited patent. As a result, the injection from the emitter is not optimally uniform on all sides into the surrounding base region. Moreover, no provision is made in the cited patent for determining the impurity concentration of the intrinsic base region independently of the extrinsic base region.